1. Field of the Invention
The present invention relates generally to telecommunications and more particularly to transmitters and receivers for fiber optic networks.
2. Background Information
In current fiber optic networks, an electronic data stream is fed to a laser amplitude modulator. The laser amplitude modulator typically pulses or alters the laser output to create an amplitude-modulated optical signal representative of the electronic data stream. The laser amplitude modulator and laser thus define a transmitter for transmitting the optical signal over an optical fiber, which is then received by a receiver. The receiver for the amplitude-modulated optical signals of the optical data typically includes a photodiode to convert the optical signals back into the electronic data stream.
The reading of the amplitude-modulated optical data signals using a photodiode is straightforward: the optical signals either produce an electric output at the photodiode or they do not. As a result, an output electronic data stream of zeros and ones is generated.
However, optical fiber may be tapped. The optical fibers can be spliced or even merely clamped so as to obtain optical signals from the fiber. It also may be possible to tap fibers without physically touching the optical fiber, for example by reading energy emanating or dissipating along the fiber. Amplitude-modulated optical signals, with their ease of detection from a photodiode, require that only a small amount of energy be tapped and passed through the photodiode in order to be converted into a tapped electronic data stream.
To confront non-secure optical and non-optical data lines, it has been known to use public key/private key encryption so that the data stream being transmitted is encoded in a format that makes it difficult to decode. Encryption however has several drawbacks, including the need for extra processing steps and time. Moreover, public key/private key encrypted data can be cracked, and the devices and algorithms for doing so are constantly improving.
U.S. Pat. No. 5,455,698 purports to disclose a secure fiber optic communications system based on the principles of a Sagnac interferometer. A data transmitter is a phase modulator for modulating counter-propagating light beams sent by a receiver round a loop. The receiver includes a light source, a beamsplitter for splitting light from the light source into counter-propagating light beams and for receiving the phase-modulated light beams, and an output detector. U.S. Pat. No. 5,223,967 describes a similar Sagnac-interferometer-based system operating over a single optical fiber.
The Sagnac-interferometer-based systems described in these patents have the disadvantage that they require the light to travel over a loop, whether back and forth in a single fiber or over a long length looped fiber. As a result, either the link budget for the single fiber must be doubled or else a looped fiber with significant and expensive extra length of at least twice that of a single fiber must be laid between the transmitter and the receiver. Moreover, the receiver contains the light source, as opposed to the current installed base where the transmitter has the light source.
The Sagnac-interferometer-based systems thus are expensive to build and operate, and do not work particularly well with existing systems.
U.S. Pat. No. 6,072,615 purports to describe a method for generating a return-to-zero optical pulses using a phase modulator and optical filter. The RZ-pulse optical signal transmitted over the fiber is easily readable by a detector.
U.S. Pat. No. 5,606,446 purports to describe an optical telecommunications system employing multiple phase-compensated optical signals. Multiple interferometric systems are combined for the purpose of multiplexing various payloads on the same optical transmission path. The patent attempts to describe a method for providing fiber usage diversity using optical coherence length properties and a complex transmit/receive system. Each transmitter has a splitter, a plurality of fibers and a plurality of phase modulators to create the multiplexed signal, which is then demultiplexed at the receiver. This system is complex and expensive.
The phase-modulated based systems described above moreover are not compatible with existing receivers, a major disadvantage.
An object of the present invention is to provide a transmitter for transmitting either phase-modulated or amplitude-modulated optical signals. An alternate or additional object of the present invention is to provide a receiver for receiving either phase-modulated or amplitude-modulated optical signals.
The present invention provides a transmitter having at least one light source, a phase modulator for phase modulating light from the light source, and a controller having an input for receiving an electronic data stream, the controller in a first mode controlling the phase modulator so as to create phase-modulated optical signals in the light as a function of the electronic data stream and the controller in a second alternate mode amplitude-modulating the light as a function of the electronic data stream.
The present invention thus permits a phase-modulated transmission mode or an amplitude-modulated transmission mode, or both a phase and amplitude modulated transmission mode, which can permit the transmitter to work with different types of receivers. An optical fiber typically connects the transmitter of the present invention to the receiver.
The controller in the first mode preferably phase-modulates the light as a function of an output of a delayed-feedback exclusive-or gate having the electronic data stream as an input. The first mode is thus a highly secure data transmission mode, as described in co-owned and co-pending U.S. patent application Ser. No. 09/765,153, entitled xe2x80x9cSecure Fiber Optics Telecommunications System and Methodxe2x80x9d and filed on Jan. 17, 2001, the entire disclosure of which is hereby incorporated by reference herein.
In the second mode, the light may be amplitude modulated either by altering the energy provided to the light source or by altering the light emitted by the light source. The light source preferably is a laser, for example a semiconductor laser operating at a 1550 nm, or other, wavelength.
In the second mode, the light may be amplitude modulated either in direct relation to an input data stream (known as the direct second or amplitude-modulated mode), or as a function of an output of a delayed-feedback exclusive-or gate having the electronic data stream as an input (known as the delayed second or amplitude-modulated mode). In the delayed second mode, the optical signal may or may not also be phase modulated. In the direct second mode, the amplitude-modulated optical signals sent by the transmitter can be read common receivers, or by the receiver of the present invention. In the delayed second mode, the amplitude-modulated optical signals can be read by the receiver of the present invention as well as by the receiver of incorporated-by-reference U.S. patent application Ser. No. 09/765,153, entitled xe2x80x9cSecure Fiber Optics Telecommunications System and Methodxe2x80x9d and filed on Jan. 17, 2001.
The controller preferably has two circuits, a first circuit for controlling the phase modulation and a second circuit controlling the amplitude modulation. Preferably, a switch, which may be composed of hardware or software, is provided to activate the first mode, the delayed second mode, or the direct second mode. An operator may set the switch of a first transmitter to the first mode, the delayed second mode, or the direct second mode, so that the transmitter generally always operates in that mode.
Alternately, the switch can be controlled by bit data in a packet of a packet-based data input stream. The bit data may be set for example to zero or one or two or three, so that the data contained in the packet is sent either via the first mode or via the direct second mode or the delayed second mode with no phase modulation or the delayed second mode with phase modulation as a function of the bit data. The transmitter thus produces an alternating amplitude-modulated and phase-modulated data stream, which can be read by a receiver of the present invention. The packets preferably contain data regarding the transmission mode for the next packet so as to permit the receiver to have time to switch between alternate receive modes.
Both the operator-set and packet-switched systems have the advantage that telecommunications providers can provide customers differentiated services, for example a secure mode and a non-secure mode, although the bit-based method provides carriers more options for devising service levels.
The at least one light source may include two lasers, a first laser for the amplitude-modulated signals, and a second laser for the phase-modulated signals. A coupler couples the light from the two lasers together. Preferably, however, a single laser directly next to the phase-modulator is provided. This prevents delay between the laser and the phase-modulator when modes are switched.
The present invention also provides a receiver for receiving optical signals, the optical signals including both phase-modulated optical signals and direct amplitude-modulated optical signals. The receiver includes an interferometer for reading the phase-modulated signals and a detector to read the direct amplitude-modulated optical signals.
The receiver also may read delayed amplitude-modulated optical signals through the interferometer.
Preferably, an energy level detector is also provided at the receiver for measuring light energy in a fiber.
Preferably, the second light path has a delay with respect to the first light path, the delay being matched to a delay at the transmitter during the phase-modulated transmission mode and the delayed second amplitude-modulated mode.
The receiver can read a mixed optical signal of both phase-modulated and direct and delayed amplitude-modulated signals, with the direct amplitude-modulated signals being read off the third path.
The receiver can be set by an operator to receive in one of the three modes, or can be switched to the various receive modes by a bit set in a packet. For example, the current packet being received sets the receiver to the proper receive mode for the next packet.
The present invention also provides a dual-mode optical transmission system comprising a transmitter for transmitting amplitude-modulated signals in a first mode and phase-modulated signals in a second mode, an optical fiber connected to the transmitter, and a receiver having an interferometer being connected to the optical fiber. The first and second mode signals can be read by the receiver, and can be switched based on either an operator input or packet-based data.
The present invention also provides a method for transmitting optical data in two modes comprising the steps of:
phase modulating light from at least one light source during a first transmission mode so as to transmit phase-modulated optical data; and
amplitude modulating light from the at least one light source during a second alternate transmission mode so as to transmit amplitude-modulated optical data.
Preferably, the at least one light source is a single laser.
The amplitude modulating step may include amplitude modulating the light as a direct function of an input electronic data stream, or as a function of an output of a delayed-feedback exclusive-or gate.
The method may further include switching between the phase modulating and the amplitude modulating steps as a function of a packet bit set.
Also provided by the present invention is an optical signal comprising amplitude-modulated signals representative of an input data stream during a first time period and phase-modulated signals representative of the input data stream during a second time period subsequent or prior to the first time period.
It should be understood however that, while phase-modulated signals are preferred in the secure transmission mode, under certain circumstances a mixture of phase and amplitude modulation could be possible. For example, amplitude modulated signals not related to the input optical data stream could be transmitted during the secure phase-modulation mode without necessarily affecting security.